In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low costs among other relevant features. While practically all existing solutions comprise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of a mains energy into the form required by the LEDs while complying with present and preferably future power mains regulations. In addition, it is required that the driver circuits compatible with existing and legacy power adjustment means, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units.
The driver circuits should comply with all kinds of dimmers and especially the drivers should comply with phase-cut dimmers, which are preferably used to regulate the mains powers with low power loss. Those dimmers which were initially designed to regulate the mains energy provided to a filament lamp utilized the low load impedance path of the filament for a timing circuit operation current to adjust the phase-cut timing. Alternatively to providing this path continuously, connecting and disconnecting this path for a certain part of the mains voltage cycle can also result in a stable operation of the dimmer. The provision of this low impedance path has to be adjusted with respect to the zero crossing of the mains voltage. To achieve timely provision of this low impedance path, the zero crossing is usually detected by the driver circuit of the lamps while it is in a high impedance state. Such a zero crossing detection is complicated and has a high technical effort and if a large amount of LED units is connected to one dimmer circuit, the technical effort increases due to the required increase of impedance of each individual LED unit.
WO 2009/121956 A1 discloses a lighting apparatus comprising an LED assembly and a rectifier unit to connect the LED unit to a dimmer circuit. The LED unit comprises a bleeder connected in parallel to the LED unit to provide a bleeding current. The bleeder unit is controlled by a control unit connected to the LEDs to provide a bleeding current at a certain point in time of the rectified AC voltage. This control unit is complicated and the power factor of the whole lighting apparatus is reduced due to the bleeding current.
US 2012/0056553 A1 discloses a driver device for connecting an LED unit to a dimmer device, wherein two parallel bleeding paths are provided comprising different resistance values in order to adjust the rectified input voltage in different parts of the main voltage cycle differently. Since the two bleeding paths are both adapted to be connected to the mains voltage, high voltage components are necessary and since the phase of the mains voltage has to be determined, this bleeding circuit is technically complicated and requires an increased amount of large components such that an integration of these components is not possible.